The metric system is an internationally agreed-upon measurement system based on decimals or powers of 10. Scientists use a refined version called the International System of Units (abbreviated SI). In...The metric system is an internationally agreed-upon measurement system based on decimals or powers of 10. Scientists use a refined version called the International System of Units (abbreviated SI). In biology, you will often find a need to describe measurements of length, volume, mass, time, temperature or amount of substance.